Two-speed pump

ABSTRACT

A stepped displacement fluid pump having a plurality of piston members reciprocable individually or in common within a single fluid chamber to displace fluid from the chamber at different rates proportional to the sum of the exposed surface areas of those piston members reciprocating in common. Coupling means carried on the piston members selectively interconnect and disconnect the piston members in different stages of operation of the device in response either to positioning the piston members at predetermined locations relative to one another or in response to the pressure of fluid in the chamber.

United States Patent [191 Craft Nov. 27, 1973 TWO-SPEED PUMP Inventor:Roger L. Craft, St. Joseph, Mo.

[73] Assignee: Gray Manufacturing Company,

Inc., St. Joseph, Mo.

[22] Filed: Oct. 1, 1971 [2]] Appl. No.:,l85,528

[52] US. Cl 417/214, 60/477, 60/486, 417/487 [51] Int. Cl. F04b 19/00[58] Field of Search 60/52 HA, 52 118, 60/477, 486; 417/212, 214, 486,487

[561' References Cited UNITED STATES PATENTS 3,112,705 12/1963Chlebowski 60/52 HA X 1,263,401 4/1918 Fraser 417/487 2,185,264 l/l940Mistral 60/52 HA 2,703,191 3/1955 Jernander 417/487 X 2,050,000 8/1936Frost 60/52 HB 2,370,681 3/1945 Mueller 60/52 HB FOREIGN PATENTS ORAPPLICATIONS 181,586 10/1966 U.S.S.R 417/487 Primary Examiner-C. J.Husar Assistant Examiner-Leonard Smith Attorney-Schmidt, Johnson, Hovey& Williams [57] ABSTRACT A stepped displacement fluid pump having aplurality of piston members reciprocable individually or in commonwithin a single fluid chamber to displace fluid from the chamber atdifferent rates proportional to the sum of the exposed surface areas ofthose piston members reciprocating in common. Coupling means carried onthe piston members selectively interconnect and disconnect the pistonmembers in different stages of operation of the device in responseeither to positioning the piston members at predetermined locationsrelative to one another or in response to the pressure of fluid in thechamber.

16 Claims, 9 Drawing Figures PAIENIEBHUVZ'! I975 3.775.027 sum 1 or 2INVENTOR. Roger L. Craff I BY TWO-SPEED PUMP This invention relatesgenerally to fluid power systems and, more particularly, to fluiddevices capable of displacing motive fluid at different rates withoutaltering the speed or stroke length of the device.

Certain applications of the use of fluid power systems ideally demandthat motive fluid flow be supplied at different combinations of flowrates and pressures. For instance, when the hydraulic actuator of thesystem is encountering minimal resistance to its movement, it is oftendesirable to deliver fluid at high flow rates in order to drive theactuator at high speeds. Power limitations, however, preclude such highspeed operation when the actuator encounters a heavy load. In thislatter instance substantial pressures must be built up to drive theactuator, and fluid must necessarily be delivered at a far slower ratein order to remain within the particular power limitations of thesystem. By providing a system capable of delivering fluid selectively athigh or low rates of flow, the power input and effort required todeliver the motive fluid can be maintained at a relatively low level,while drastically altering the operating speed and force output of theactuator to match the changing resistance encountered.

A typical example of such a system is a hydraulically powered liftingjack which travels unhindered through a substantial portion of itsstroke before engaging a heavy object to be lifted. A two-stagehydraulic system as described above, will drive the jack at lowpressures and high speeds when unloaded, and will drive the jack atslower speeds and higher pressures after the object is engaged. Jackcycling speed is, therefore, maximized without limiting the liftingcapabilities of the jack; yet, the effort required to supply motivefluid and operate the jack remains at a level that is amenable to directoperation by a limited manual force.

One prior form of such a hydraulic system capable of operating atmultiple combinations of speeds and pressures, contemplates a pair ofseparately operated piston type pumps delivering fluid to the actuatorin different stages. The volume of fluid displaced by each pump is equalto the effective cross-sectional working area of its pumpingelementexposed to the fluid, multiplied by the stroke length of thepumping element. One pump is sized to deliver a high volume of motivefluid to the actuator and functions while the actuator in unloaded; theother pump has smaller displacement, either a shorter stroke length orsmaller exposed piston working area, to deliver a relatively smallvolume of fluid and functions when the actuator encounters heavyresistance. Such arrangement, of course, requires duplicate fluidpressure feed systems and pump operating handle members, as wellasadditional hydraulic controls that efl'ect operation of the properpump during each stage of operation.

Another prior art form of a multiple speed fluid power systemincorporates a pair of separate pumps which are operated in tandem inall stages of pump operation. A low pressure relief valve in the outputflow of one of the pumps opens whenever heavy resistance is encounteredand high pressure developes to relieve and divert fluid displaced fromthat pump to a reservoir so that only fluid from the other non-relievedpump performs work on the actuator during a slower speed operatingstage. The displacement of the two pumps effectively add together whenoperating in tandem to drive the actuator at high speed, therebyreducing the necessary size of at least one of the pumps. A singleoperating handle is operably included to drive the pumps in equal strokelengths so that the work performed by the displaced fluid is directlyproportional to the sum of the working areas of the pumps during tandemoperation, and is directly proportional only to the working area of thenon-relieved pump during the slow speed operating stage. While thistandem arranged is somewhat simpler than the separate pump system abovedescribed, it still incorporates a number of duplicate, likefunctioningelements. In addition, this type of arrangement has an inherentinefficiency in that fluid displaced from the relieved pump performsuseful work during only the high speed stage of operation of the system,while effort expended in displacing that fluid during the low speedstage of operation is completely wasted.

It would be highly advantageous, therefore, to provide a single fluiddevice capable of delivering fluid at various combinations of pressuresand rates of flow wherein all fluid displaced from the device performsuseful work.

Accordingly, it is an important object of the present invention toprovide a staged operating fluid device capable of displacing fluid atdifferent flow rates in response to different pressures of the fluiddelivered wherein all of the fluid displaced during all stages ofoperation performs useful work in order to minimize inefficient energyloss by the device.

Another important object of the invention is to provide a fluid devicehaving a fluid chamber and piston means movable therein for displacingfluid therefrom to a single discharge duct directing the fluid throughexternal circuitry to perform useful work, wherein the piston meansinclude separate pistons movable individually or in common in thechamber to vary the exposed working area of the piston means in relationto the number of commonly moving pistons so that flow through the singledischarge duct may be altered in corresponding stages without inducingsubstantial energy loss and without requiring incorporation of amultiplicityof like functioning elements.

Another object of the present invention is to provide a reciprocatingpiston pump having piston means including a primary working pistonmovable in a fluid chamber to displace fluid therefrom, and an accessoryworking piston that can be selectively coupled to the primary piston toincrease the effective working area of the piston means when coupled, sothat the pump displaces a higher rate of fluid when the pistons arecoupled than when uncoupled without requiring alteration of the strokelength of the pump or speed of operation of same.

A more particular object of the invention is to provide a pumpconstructed in accordance with the preceding object and further providedwith pressure responsive coupling means which are engageable with theprimary and accessory pistons to couple same when the pressure of motivefluid in the chamber is below a predetermined level so that the pump candisplace said high rate of fluid flow at low pressures in a firstoperational mode, said pressure responsive coupling means being operableto automatically disengage and uncouple the pistons during movement ofthe primary piston while chamber pressure is above the predeterminedlevel so that the pump displaces a low rate of fluid flow at highpressures in a second operational mode.

Another important object of the invention is to provide a fluid deviceas constructed in accordance with any of the preceding objects thatfurther includes sealing means operable to seal both statically anddynamically between the relative moving pumping members, and between thewalls of the fluid chamber and the piston means, in order to preventfluid leakage from the chamber during all operational modes of thedevice.

A more particular object of the invention is to provide a pumpconstructed in accordance with the preceding object which utilizes aunitary seal member that effects both the sealing between the severalpiston members, as well as the sealing between the chamber walls and thepiston means, in order to avoid inclusion within the device of amultiplicity of similarly functioning sealing pump components.

Another important object of the invention is to provide a reciprocatingpiston pump having a primary working piston and an accessory workingpiston selectively movable in common with the primary working piston todisplace fluid at different rates when coupled and uncoupled, andposition responsive coupling means selectively interconnecting theprimary and accessory pistons upon positioning same in a predeterminedcoupling position relative to one another to displace fluid in directproportion to the sum of the exposed working areas of the primary andaccessory pistons, said coupling means being operable to disengage uponmoving one of the pistons away from said relative coupling position topermit the latter piston to reciprocate alone and displace fluid indirect proportion to its working area so that the displacementcapability of the pump changes as the coupling means engage anddisengage.

A further object of the invention is to provide a piston pumpconstructed in accordance with the preceding object wherein saidposition responsive coupling means are also responsive to the pressureof motive fluid displaced by the pump in such a manner as toautomatically disengage during movement of said one piston in responseto a rise in pressure above a predetermined level to permit saidreciprocation of the one piston alone in order to automatically reducethe effort required to operate the pump at high pressures.

These and other objects and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of a preferred embodiment of the invention andseveral modifications thereof, together with the accompanying drawings,wherein:

FIG. 1 is an elevational view of a hydraulically operated lifting jackprovided with an actuating fluid pump constructed in accordance with thepresent invention;

FIG. 2 is a top elevational view of the actuator assembly mounting blockwith the pump and actuator removed;

FIG. 3 is a longitudinal, cross-sectional view of the mounting blocktaken along line 33 of FIG. 2;

FIG. 4 is a longitudinal, cross-sectional view taken along line 44 ofFIG. 2 showing the mounting block and actuator assembly;

FIG. 5 is a longitudinal, cross-sectional view taken generally alongline 5-5 of FIG. 2 showing the mounting block and improved pump;

FIG. 6 is a longitudinal, cross-sectional view of a modified form of thefluid device as contemplated by the invention;

FIG. 7 is a longitudinal, cross-sectional view similar to FIG. 6 ofstill another modified form of the present invention;

FIG. 8 is a longitudinal, cross-sectional view of yet another modifiedform of the invention and illustrating the low volume mode of operationthereof; and

FIG. 9 is a view similar to FIG. 8 illustrating the high volume mode ofoperation.

FIGS. 1-5 illustrate a lifting jack assembly 10 having an integrallycarried fluid power system which includes a fluid device in the form ofa supply pump 12, constructed in accordance with the present invention.The fluid power system also includes a ram-type hydraulic actuatorassembly 14 that is supplied with fluid from pump 12 to operate thejack. The overall construction and operation of the jack assembly andfluid power system will be first described.

The jack assembly 10 has a scissors frame composed of crossed, pivotallyinterconnected arms 16 and 18 which respectively open and close inscissors-like action to effect raising and lowering of the frame arm 16.A lifting pad 20 is provided at the upper end of arm 16 so as toliftingly engage and disengage a vehicle or other heavy object as thearms 16 and 18 rotate in opposite directions about frame pivot 22.

The fluid power system is pivotally connected to frame arm 18 at pivot26 and pivotally connected to frame arm 16 at pivot 28. A mounting block30 carries the supply pump 12 and actuator assembly 14 as a unit infixed relationship to the jack frame through its connection at pivot 26to frame arm 18, while the outer end of a ram 32 depending out of theactuator assembly 14 is mounted on frame arm 16 by pivot 28. Duringoperation of the jack, ram 32 is driven outwardly and downwardly withrespect to FIGS. 1 and 4 to effect raising of lift pad 20, and retractsback into actuator assembly 14 to effect opposite rotation of the framearms to lower the lift pad.

The actuator. assembly 14 includes an external, longitudinal cylinder 42affixed upon the upper face 31 of the mounting block 30 and an innercylinder 44 concentrically arranged with respect to the externalcylinder 42 and threadably secured to the mounting block 30 at a throughbore 34 therein. An end cap 46 is rigidly secured to the upper end ofinner cylinder 44 and sealingly engages the walls of external cylinder42 so as to form the upper ends of both cylinders 42 and 44. The innercylinder 44 has an internal, longitudinal fluid working chamber 48,while the concentric cylinders 42 and 44 define an annular reservoirchamber 50 therebetween disposed concentrically to the actuator workingchamber 48. The end cap 46 is provided with an internal passage 52communicating with the internal working chamber 48 and annular reservoirchamber 50, with portions of internal passage 52 extending through avalve body 54 affixed within end cap 46. The outer cylinder 42 has aremovable plug 72 facilitating the filling of the fluid reservoirchamber 50.

A valve stem 56 is threadably received within valve body 54 and has amanually operable knob 58 secured thereto. Upon rotation of knob 58, aneedle valve 60 carried at the inner end of valve stem 56 is drivenbetween positions blocking and permitting flow through internal passage52. Needle valve 60 acts as a manual pressure release valve, blockingcommunication between low pressure reservoir 50 and working chamber 48in the position shown in FIG. 4 to permit pressurization of chamber 48and operation of the actuator. When moved away from its non-flowblocking position, needle valve 60 permits return fluid flow fromworking chamber 48 to low pressure reservoir 50 to allow retraction ofram 32.

Ram 32 is received within working chamber 48 to'be responsive to thepressure of motive fluid therein. A bearing guide 62 at the inner end ofram 32 cooperates with the inner wall of cylinder 48 to guide the ram inlongitudinal reciprocation within chamber 48. Sealing means 64 at thelower end of mounting block 30 in the area of bore 34 cooperate with ram32 to prevent fluid leakage from working chamber 48. Ram 32 is slightlysmaller than bore 34 to assure constant fluid communication of thelatter with the fluid chamber 48 proper. Ram bearing guide 62 has aplurality of passages 66 extending therethrough to interconnect theportion 68 of working chamber 48 disposed between the upper end surfaceof bearing guide 62 and end cap 46, with the annular portion 70 ofworking chamber 48 defined between the main body of ram 32 and theinternal wall of cylinder 44. Upon introduction of pressure fluid flowinto chamber 48, pressure therein acts upon the upper surface of bearingguide 62 to force ram 32 outwardly and downwardly from working chamber48. During retraction of ram 32, fluid is displaced from working chamber48 across the opened needle valve 60 through internal passage 52 to lowpressure reservoir chamber 50.

Pump 12 (described in greater detail below) has a housing 74 secured tomounting block 30, and an actuator rod member 76 with an inner enddisposed within an internal longitudinal fluid chamber 78 formed as aconstant diameter bore in housing 74. A duct 80 connects the fluidchamber 78 with a passage 82 in mounting block 30. Passage 82, in turn,communicates with separate fluid inlet and outlet passages 84 and 86. Aone-way inlet check valve 36 intercepts inlet passage 84 and isspring-biased to a closed position shown in FIG. 5, blockingcommunication from inlet passage 84 to passage 82. A one-way outletcheck valve 38 disposed in outlet passage 86 is similarly biased to itsclosed position, blocking flow from passage 82 to outlet passage 86.Inlet passage 84 extends to the upper face 31 of mounting block 30 tocommunicate with annular reservoir chamber 50, and outlet passage 86opens into bore 34 of mounting block 30 so as to communicate withactuator working chamber 48 at all times.

General operation of the jack assembly proceeds as follows. Upon upward,suction stroke travel of actuator rod member 76, pressure in chamber 78and passage 82 drops to a very low level permitting inlet check valve 36to move to an open position in opposition to its spring-bias so tthatfluid may be drawn from annular reservoir 50 through inlet passage 84 topump fluid chamber 78. Upon downward travel of actuator rod member 76,pressure increases sufficiently to close inlet check valve 36 andovercome the bias of outlet check valve 38 and effect opening of same.Fluid displaced from pump chamber 78 is accordingly across outlet checkvalve 38 to outlet passage 86 and the actuator working chamber 48. Aseparate pressure relief passage 88, shown in FIG. 3, extends betweenthrough bore 34 and the upper face 31 of mounting block 30 to permitselective communication between actuator working chamber 48 and annularreservoir 50. A high pressure relief valve 40 of conventional structure,in-

tercepts relief passage 88 to permit flow therethrough only whenexcessive pressure develops within working chamber 48 in order toprotect the actuator from over-pressurization and resultant injurythereto.

The jack is operated in a power-lifting stroke by pumping handle 24 torepeatedly cycle pump actuator rod member 76 pivotally connected theretoby a connecting link 90. The pump alternately draws fluid from reservoirchamber 50 from inlet passage 84 and pumps the same through outletpassage 86 to actuator working chamber 48 to move ram 32 downwardly ashort distance to a new position upon each cycle of the handle and rodmember 76. Outlet check valve 38 prevents reverse flow from workingchamber 48 during the pump suction stroke to hold the ram in its newposition while awaiting the next pumping stroke. The jack frame arms 16and 18 rotate in a scissors opening movement about frame pivot 22 toraise lifting pad 20 and the object engaged thereby.

To collapse the jack and lower the lift pad 20, valve knob 58 is rotatedon drive needle valve upwardly and clear end cap passage 52. The loadexperienced by ram 32 drives it upwardly in work chamber 48 to displacefluid therefrom through cleared passage 52 to reservoir 50. Ram 32 thusretracts back into cylinder 44 to lower liftpad 20 and collapse the jackframe. The ram may be held in a desired position by moving needle valve60 to its closed position to isolate working chamber 48 andhydraulically lock ram 32.

Turning now particularly to the improved fluid pump 12 which is thesubject of the present invention, pump 12 has a centrally located rodpumping member 76 of substantially smaller diameter than the constantdiameter of the pump fluid working chamber 78. A seal pack comprising anelastomer seal 92 and rigid backup washers 94 and 96, one of which ismetallic, is disposed in the annular ring area defined between rod 76and the cylindrical wall of chamber 78 to sealingly engage both rod 76and the chamber wall. A tubular coupling sleeve is disposed within theinternal chamber of housing 74 and extends between seal backup washer 96and the upper end surface 102 of the housing interior chamber 78. Rod 76has a pair of spaced, annular shoulders which carry a compressibledetent ring 106 therebetween in sliding relationship to the interiorsurface of tubular sleeve 100. The tubular sleeve 100 has an indentedridge 108 near its upper end that protrudes inwardly into interferingrelationship with compressible detent ring 106.

Whenever actuator rod 76 moves upwardly during its suction stroke,detent ring 106 will ultimately contact ridge 108, whereupon slightlygreater force must be exerted to pull actuator rod 76 farther upwardlyand simultaneously radially compress detent ring 106 to permit thelatter to travel upwardly past sleeve ridge 108. When so located aboveridge 108 in what may be termed a coupling position, detent ring 106cooperates with the sleeve ridge to exert a downward force on sleeve 100and seal 92, tending to carry the latter downwardly in commonreciprocation with the rod. A snap ring shoulder 110 at the lower end ofrod 76, contacts the bottom of seal 92 when detent ring 106 is locatedabove ridge 108 in this coupling position to effect common upwardreciprocation of the seal 92 and rod 76. Accordingly, when rod 76 islocated in this coupling position relative to sleeve 100 and seal 92,the actuator rod 76 and seal 92 will reciprocate in common to displacefluid from chamber 78 at a rate directly proportional to the sum of thearea of exposed working surface 112 of the rod, and the annular area ofthe lower exposed surface 114 of seal 92. It will be apparent thatduring such common reciprocation, fluid is displaced at a rateproportional to the entire diameter of chamber 78 in view of the sealingengagement of seal 92 with the walls 98 of chamber 78.

Rod 76 is also adapted to reciprocate alone within chamber 78 andrelative to seal 92 while the latter remains in stationary relationshipto housing 74. This operational mode of the pump is effected wheneverthe frictional securement between detent ring 106 and ridge 108 isbroken and the detent ring 106 is positioned below ridge 108 in themanner depicted in FIG. 5. The outer surface of rod 76 slides upon theannular inner surface of seal 92 when reciprocating alone in chamber 78in what may be termed the low speed-high pressure operating stage ofpump 12. In this operating stage it will be apparent that only theexposed surface 1 12 of rod 76 is moving within chamber 78 so that fluidis displaced from the chamber at a rate directly proportional to thecross-sectional area of surface 112. A lesser volume of motive fluid isthereby displaced from the pump during each full stroke of rod 76 thanis displaced when seal 92 reciprocates in common with rod 76. Thoughactuator ram 32 operates at a slower rate during this low volumeoperating mode, all effort expended by stroking handle 24 isconcentrated upon the smaller fluid volume displaced, enabling thissmaller volume to be displaced at a substantially higher pressure tocreate a correspondingly greater lifting force, all this without wastingeffort expended on handle 24. The pump 12 is particularly adaptable tomanual operation therefore, since a limited manual power input caneffect rapid rise of lifting pad 20 when the same is unloaded by drivingthe rod 76 and seal 92 in common reciprocation, and since this limitedinput power can also be concentrated to deliver smaller volumes ofhigher pressure fluid to exert greater lifting force by driving only therod 76 in reciprocation in fluid chamber 78. In the embodimentillustrated in FIG. 5, the crosssectional diameter of rod surface 112 isapproximately one-half of the combined diameter of surfaces 112 and 114so that the pump displacement is four times as great when rod 76 andseal 92 move in common reciprocation than when rod 76 reciprocatesalone.

Whenever rod 76 is positioned in the coupling position described above,the frictional holding force exerted between compressible detent ring106 and indented ridge 108 is effectively opposed by the hydraulic forcecreated from pressure of fluid in chamber 78 exerted on seal 92 toresist downward travel of the latter. The magnitude of the frictionalholding force exerted between detent ring 106 and ridge 108 isdetermined by the rigidity of detent ring 106 as well as the distance ofradial interference between ridge 108 and detent ring 106, the latterdistance factor controlling the amount of radial compression required ofdetent ring 106 in traveling across ridge 108. The pump 12 isconstructed so that the hydraulic resisting force exerted upon seal 92will exceed the frictional holding force of the detent ring wheneverpressure within chamber 78 rises above a predetermined operating level.In the preferred embodiment this predetermined pressure level is of amagnitude representing engagement by pad 20 of a load to be lifted,i.e., whenever chamber pressure is below this predetermined level, thejack is unloaded and amenable to high speed operation; while uponengagement of a load, pressure in chamber 78 rises above thispredetermined level.

Upon forcing actuator rod 76 downwardly in its power stroke whilepressure in chamber 78 is above this predetermined level, the hydraulicresisting force overcomes the frictional intersecuring force tending tourge the sleeve and seal 92 downwardly. Accordingly, the seal and sleevewill remain in their upper positions illustrated in engagement withupper chamber end surface 102, and detent ring 106 will compress andtravel across ridge 108 to uncouple the seal and rod upon forcing rod 76downwardly in its power stroke. Rod 76 is thereby released toreciprocate along within the chamber 78 to effect the low speed-highpressure operational mode of the pump. In this manner, the rod isautomatically uncoupled from sleeve 100 and seal 92 whenever the rod isstroked while pressure in chamber 78 is above the predetermined leveland offering resistance greater than the frictional intersecuring forcecreated by detent ring 106. Thus, for instance, if the jack comes intolifting contact with a heavy load during midstroke of rod 76, pressurein chamber 78 will instantly rise above the predetermined level and thedetent ring will immediately uncouple to switch the pump to the lowpressure-high flow operational mode and permit completion of the stroke.

Seal member 92 is operable to effect a static seal with rod 76 and todynamically seal against the walls of fluid chamber 78 during commonreciprocation with the rod, and is conversely operable to staticallyseal against the chamber walls and dynamically seal against the rod whenthe rod 76 reciprocates relative to seal 92. A conventional V-shaped lipelastomer seal with concentric, radially compressible lips at its innerand outer annular surfaces has been found quite suitable for use in theimproved pump. The relatively rigid backup washers prevent extrusion ofcompressible seal 92 and provide sufficient support therefor to permitthe seal pack to function as a pressure fluid displacing pumping member.

The rod 76 is reciprocable through a full stroke length delimited in onedirection by engagement of upper shoulder 104 with upper chamber endsurface 102, and delimited in the opposite direction by engagement ofrod lower surface 112 with the lower end surface 116 of the fluidchamber 78. In the preferred form shown in FIG. 5, the ridge 108 anddetent ring 106 are positioned respectively upon sleeve 100 and rod 76relative to one another and to the chamber end surfaces 102 and 116 soas to permit the rod to travel throughout its full stroke length bothwhen coupled and uncoupled from sleeve 100. Rod 76 is permitted slightovertravel in its upward suction stroke relative to the upward travel ofsleeve 100 in order to move the coupling position described above;however, the rod is still movable substantially throughout its fullstroke length either when reciprocating alone or in common with seal 92since the rod must be moved essentially to the end of its stroke beforereaching its coupling position.

The modified fluid pump illustrated in FIG. 7 incorporates a pair ofconcentrically arranged piston elements 132 and 134 reciprocablydisposed within an interior fluid chamber 138 of housing 140. Seals 156are carried on piston elements 132 and 134 to effect dynamic and staticsealing between the pistons and between the outer piston 134 and thechamber walls. The

centrally disposed piston element 132 acts as the pump actuator rod andcarries a compressible detent ring 142 movable to opposite sides ofannular ridge 144 on the adjacent, concentric piston member 134. Whenpositioned relative to ridge 144, as shown in FIG. 7, the detent ring142 exerts a predetermined frictional securing force actingto carry thepiston member 134 in common reciprocation with actuator piston 132. Whendetent ring 142 is disposed below ridge 1'44, piston member 132 is freeto reciprocate alone inchamber 138.

The position-coupling, pressure-uncoupling and consequent relative andcommon reciprocation of piston members 132 and 134 is analogous to thatdescribed above with respect to actuator rod 76 and seal 92, andtherefore, will not be set forth again in detail. Suffice it to say,that fluid will be displaced from chamber 138 at rates proportional tothe exposed working area of surface 146 of piston member 132 duringrelative, alone reciprocation of piston member 132, and at ratesproportional to the sum of area 146 and the exposed working area ofsurface 148 of piston member 134 during common reciprocation.

The detent mechanism of FIG. 7 includes a compressible, annular detentring 142 preferably formed of slightly resilient plastic material.Detent ring 142 is longitudinally spaced from an enlarged diameter endportion 158 of the central piston member 132 to define a space betweendetent ring 142 and end portion 158 for accepting the indented ridge 144of annular piston 134. Ridge 144 is positionable in the space betweendetent ring 142 and end portion 158 by forcing slight overtravel ofpiston member 132 in the upward direction in a manner analogous to thecoupling-efi'ecting overtravel of rod 76 described with respect to theembodiment of FIGS. 1-5.

The modified form of the invention depicted in FIG. 6 of the drawings isquite similar to FIG. 7 but incorporates a third, concentric, annularpumping member 136 disposed within chamber 138 intermediate the pistonmember 134 and the walls of chamber 138 to present a triple-steppeddisplacement pump. Pressurevof fluid in chamber 138' biases pistonmember 136 to its uppermost position shown in contact "with snap ringstop member 150 afiixed to housing 140'. Piston member 136 has anannular ridge 152, similar to ridge 144, protruding radially inwardlytoward piston member 134 to be in interfering relationship with acompressible detent ring 154 carried on the exterior of middle pistonmember 134. When the piston members are positioned as shown in FIG. 6,the inner members 132 and 134 are adapted to reciprocate in common andrelative to outer piston member 136. Common reciprocation of all threepiston members can be realized upon pulling piston members 132 and 134upwardly to compress detent ring 154 and force same over ridge 152 to aposition above the latter. The consequent interfrictional securement ofdetent ring 154 and ridge 152 will urge the outer piston member toreciprocate in common with the two inner piston members 132 and 134.Alone reciprocation of rod132 is again realized upon positioning detentring 142 below ridge 144.

In the above manner, it will be apparent that the FIG. 6 embodimentpresents a pump having three modes of operation, wherein displacement ofthe pump is correspondingly proportional to the number of piston membersmoving in common. Detent ring 154 is selectively sized to exert africtional holding force upon ridge 152 smaller than the correspondingfrictional force exerted by detent ring 142 upon ridge 144 so that theouter piston will be automatically uncoupled when pressure in chamber138 rises above a first predetermined level, and piston member 134 willsubsequently uncouple when pressure rises above a higher, secondpredetermined level during downward travel of piston member 132. Aspressure increases, therefore, pump displacement capability reduces insequential steps.

An O-ring seal 156 is carried upon the exterior surface of each of thethree piston members to seal against the adjacent interior surface ofthe adjacent piston member. Seal 156 on outer piston member 136, ofcourse, sealingly engages the walls of fluid chamber 138. Theembodiments of FIGS. 6 and 7 therefore illustrate that any number of aplurality of piston members may be used to produce a correspondingnumber of operating modes wherein the pump exhibits differingdisplacement capabilities.

Another modified version of the invention illustrated in FIGS. 8 and 9includes a housing 160 threadably secured to a fluid-conducting block162 and an end cap 166 closing one end of a fluid working chamber 164within housing 160. An actuator rod piston member 168 is longitudinallyreciprocable within a central bore in end cap 166. Carried withinchamber 164 is an annular sleeve stop shoulder 170 disposed eithertightly or loosely within the chamber that has a lower face 172 spacedlongitudinally from the end cap lower surface 174. A coupling and detentshoulder member in the form of snap ring 176 is carried upon rod pistonmember 168 so as to operatively engage an annular, ringshaped pistonmember 178 concentrically disposed with respect to the central rodpiston member 168. Below and adjacent piston member 178 is an annular,V-shaped elastomer seal member 180 similar to seal 92 of FIG. 5, and aload-bearing metal washer 182. A compression spring 184 extends betweenthe stop shoulder formed on block 162 and the load-bearing washer 182 toprovide a mechanical biasing force urging the washer 182, seal 180 andpiston member 178 upwardly into engagement with the sleeve end surface172.

Rod 168 is illustrated in FIG. 8 as being located in a coupling positionrelative to piston member 178 wherein snap ring 176 engages an upperdetent surface of member 178. Rod 168 is capable of overtravel in anupward direction relative to piston member 178 when the latter isengaging sleeve end surface 172 to move away from the relative couplingposition to the phantom position of FIG. 8. In traveling through thisfirst portion of its longitudinal stroke, rod 168 effects a first modeof operation of the pump wherein snap ring 176 remains spaced from theupper detent surface of piston member 178. Rod 168 freely slides throughthe central opening of the piston member 178 to reciprocate alone withinthe chamber 164 and displace fluid therefrom at rates proportional tothe cross-sectional area of the lower surface of piston member 168exposed to the chamber.

Rod piston member 168 is also reciprocable through a second, lowerportion of its stroke as illustrated in FIG. 9, wherein snap ring 176engages and carries piston member 178 in common reciprocation with therod member 168. It will be apparent that piston member 168 remains inits coupling position relative to the annular piston member 178throughout this lower portion of its stroke, and forces the annularpiston member 178 downwardly to displace fluid in opposition to theurgings of spring-biasing member 184. The frictional interengagement ofsnap ring 176 with the upper detent surface of piston member 178, inconjunction with pressure in chamber 164 and the bias of spring 184,frictionally holds and carries piston member 178 with piston member 168.During upward, suction stroke travel of rod 168, the biasing member willurge the annular piston member 178 upwardly in common reciprocationtherewith until the sleeve end surface 172 is engaged. In this stage ofoperation, fluid is displaced from chamber 164 at rates directlyproportional to the sum of the cross-sectional surface areas of pistonmembers 168 and 178 exposed to the chamber.

Similar to the operations of the previously described embodiments, thepump of FIGS. 8 and 9 is adapted to displace a high volume of lowpressure fluid when moving through the lower portion of its stroke ofFIG. 9, and adapted to displace smaller volumes of higher pressure flowwhen operating in the upper portion of its stroke as shown in FIG. 8. Itwill be apparent that the higher volume operation depicted in FIG. 9 canbe effected by a limited operating force on actuator rod 168 only ifpressure within chamber 164 is below a predetermined level. Forinstance, assuming rod 168 is stroked by a direct manual force, theresisting force offered by pressure 164 and acting upon both pistonmembers will ultimately exceed this inherently limited input force aspressure builds within chamber 164 and rises above a certain level. Thispressure level is, of course, directly related to the magnitude of theinput force and denotes the point when the hydraulic resisting forceexceeds the limited strength of the manual operator. The pump can,therefore, operate in its high volume mode only when pressure chamber164 is below a certain level related to the magnitude of the inputforce. Above this pressure level, of course, rod 168 is stillreciprocable through the upper portion of its stroke shown in FIG. 8 toeffect displacement of lower volumes of higher pressure fluid.

While the preferred embodiment and three modifications thereof have beendescribed above in great detail for explanatory purposes, it will beapparent that other modifications and alterations to the invention maybe made by those skilled in the art. The foregoing detailed description,therefore, is to be considered exemplary. in nature and not as limitingto the scope and spirit of the invention as set forth in the appendedclaims.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

1. A stepped displacement fluid device comprising:

a housing provided with a fluid pressure chamber and duct means foraccommodating flow of fluid to and from said chamber;

piston means reciprocably mounted in said chamber and having a workingsurface exposed to the pressure of fluid in said chamber, said pistonmeans being operable to reciprocate alone in said chamber when saidchamber pressure is above a predetermined level for displacing fluidtherefrom to said duct means in relationship to the area of said exposedworking surface;

unitary piston sealing means disposed about said piston means in sealingengagement with the walls of said chamber;

pressure actuatable coupling means for selectively engaging said pistonsealing means to thereby effeet said common reciprocation, said pistonsealing means being adapted to engage the walls of said chamber toeffect a static seal therebetween while simultaneously providing adynamic seal between said piston and sealing means as said piston meansreciprocates alone, and operable to engage said piston means to effect astatic seal therebetween while simultaneously providing a dynamic sealbetween said sealing means and said chamber walls as said piston andpiston sealing means reciprocate in common; and

actuator means operatively connected to said piston means forreciprocating therewith during the reciprocation of said piston meansalone and during said common reciprocation of said piston and pistonsealing means.

2. A stepped displacement fluid device comprising:

a housing provided with a single fluid chamber and duct means foraccommodating flow of motive fluid to and from said chamber;

piston means reciprocably mounted in said chamber in sealing engagementwith said housing for displacing fluid from said chamber in proportionto the working area of said piston means exposed to pressure in saidchamber and reciprocating therein, said piston means including first andsecond members selectively reciprocal together in common movement todisplace fluid in proportion to the sum of the working areas of saidfirst and second members exposed to pressure in said chamber andoperable to move relatively to displace fluid in proportion to saidexposed working area of the first member;

a single, pressure actuatable, selectively engageable coupling means onsaid piston means for operatively interconnecting said first and secondmembers to effect said common movement thereof when said coupling meansare engaged, said coupling means being the sole coupling force betweensaid members and operable to permit said relative movement of themembers when said coupling means are disengaged;

stop means on said housing for engaging said second member to limitmovement thereof in one direction when engaged therewith, said firstmember being positionable in a coupling position relative to said secondmember while the latter is in said stopengaging position to effectengagement of said coupling means and movable relative to said secondmember away from said coupling position while said second member is insaid stop-engaging position to effect disengagment of said couplingmeans; and

actuator means operatively connected to said first member for movingtherewith during both said relative movement and said common movement ofsaid members.

3. The device of claim 2, wherein said second member is responsive tothe pressure of fluid in said chamber to be biased thereby toward saidstop-engaging position, and wherein said coupling means comprise detentmeans associated with each of said first and second members operable tofrictionally intersecure said members for said common reciprocation uponpositioning said first member in said relative coupling position, saiddetent means being located at fixed positions on said first and secondmembers so as to disengage upon movement of said first member andassociated detenting lip away from said coupling position while saidpressure responsive second member and associated detenting lip remain insaid stop-engaging position.

4. A stepped displacement fluid device comprising:

a housing provided with a single fluid pressure chamber and duct meansfor accommodating flow of fluid to and from said chamber;

first piston means reciprocably mounted in said chamber and having afirst working surface exposed to the pressure of fluid in said chamber,said first piston means being operable to reciprocate alone in saidchamber when said chamber pressure is above a predetermined level fordisplacing fluid therefrom to said duct means in relationship to thearea of said exposed first working surface;

second pressure responsive piston means reciprocably mounted in saidchamber and having a second working surface exposed to said chamberpressure, said second piston means being adapted to move in commonreciprocation with said first piston means only when said chamberpressure is below said predetermined level for displacing fluid to saidduct means and performing work in relationship to the sum of the areasof said first and second exposed surfaces;

a single, pressure actuatable coupling means carried by one of saidpiston means for selectively interengaging said first and second pistonswhen said chamber pressure is below said predetermined level to effectsaid common reciprocation, said coupling means being the sole couplingforce between said pistons and acting in response to a rise in saidchamber pressure above said predetermined level to completely disengagesaid first and second piston means; and

actuator means operatively connected to said first piston means forreciprocating therewith during both said reciprocation of said firstpiston means alone and during said common reciprocation of said firstand second piston means.

5. The device of claim 6, wherein said coupling means comprise ashoulder on said rod disposed to engage said sleeve to carry the latterwith said rod during one portion of travel of said rod to effect saidcommon reciprocation during said portion of rod travel, said shoulderdisengaging from said sleeve to allow said relative reciprocation of therod during another portion of rod travel.

6. The device of claim 1, wherein said first piston means includes acylindrical rod having an inner end disposed in said chamber definingsaid first exposed working surface and an outer end extending outwardlyof said chamber defining said actuator means, and wherein said secondpiston means includes a cylindrical sleeve mounted to reciprocatelongitudinally in said chamber, said sleeve being positioned surroundingsaid rod and defining an annularly shaped second exposed workingsurface, said rod operable to reciprocate longitudinally relative tosaid sleeve during said reciproca tion of said rod alone.

7. The device of claim 6, wherein said second piston means furthercomprises a second cylindrical sleeve defining an annularly shaped thirdworking surface exposed to said chamber pressure, said second sleevebeing operable in response to said chamber pressure to move in commonreciprocation with said first sleeve when said chamber pressure is belowa second predetermined pressure level lower than said firstpredetermined pressure level to thereupon effect fluid displacement indirect proportion to the sum of the areas of said first, second andthird exposed surfaces.

8. The device of claim 6, further comprising means carried by said firstpiston means for dynamically sealingly engaging said sleeve and rod toprevent fluid leakage therebetween during said relative reciprocation ofthe rod and operable to effect a static seal therebetween during saidcommon reciprocation of the rod and sleeve.

9. The device of claim 8 further comprising second sealing means carriedby said second piston means for sealingly engaging the walls of saidchamber and said sleeve to prevent fluid leakage therebetween.

10. The device of claim 6, wherein said coupling means comprisenoncompressible and compressible detents located upon adjacent surfacesof said rod and sleeve and being interengageable to exert apredetermined friction force interconnecting said rod and sleeve toeffect said common reciprocation, said detents being positioned relativeto said chamber so that during movement of the rod, said chamberpressure effectively exerts a force on said detents of opposing saidpredetermined friction force and exceeding same when chamber pressure isabove said predetermined level to thereupon effect disengagement of saiddetents and permit said relative reciprocation of the rod.

11. The device of claim 10 further comprising a first stop membercarried on said housing and located to engage said sleeve to limitmovement thereof in a first longitudinal direction, said rod beingmovable in said first direction to a coupling position relative to saidsleeve to effect interengagement of said detents when said sleeve isengaging said stop member.

12. A stepped displacement fluid device comprising:

a housing provided with a single fluid pressure chamber and duct meansfor accommodating flow of fluid to and from said chamber; a cylindricalrod reciprocably mounted in said chamber and of a diameter to present acylindrical space between the rod and the chamber, said rod beingpositioned to travel longitudinally of said chamber l to effectdisplacement of fluid therefrom in propor- 3 tion to said rod diameter;unitary piston sealing means of annular configuration disposed in saidcylindrical space of the chamber T for operatively engaging the walls ofsaid chamber and said rod to prevent fluid leakage from the chamber,said piston sealing means being selectively reciprocable in common withsaid rod to ef- 1 fect fluid displacement from said chamber inproportion to said chamber diameter, said rod also being reciprocablerelative to said sealing means to effect said fluid displacementproportional to said rod diameter; and a single, pressure, actuatablecoupling means on said 1 rod for selectively engaging said pistonsealing 1 means to thereby effect said common reciprocation thereof andsaid fluid displacement proportional to said chamber diameter upon rodtravel,

said coupling means being selectively disengageable from said pistonsealing means to thereby effect said relative reciprocation thereof andsaid fluid displacement proportional to said rod diame ter upon rodtravel, said piston sealing means being operable to statically anddynamically seal against said rod during said common and said relativereciprocation of said rod respectively.

13. The device of claim 12, wherein said piston sealing means include aunitary fluid seal of annular configuration having an inner annularsurface positioned in dynamic and static sealing engagement with saidrod and an outer annular surface positioned in dynamic and staticsealing engagement with said chamber walls.

14. The device of claim 12, wherein said coupling means comprises adetent member carried on said rod operable to frictionally engage saidpiston sealing means to secure the latter to said rod upon positioningthe rod at a predetermined coupling position relative to said pistonsealing means.

15. The device of claim 14, wherein said detent member is arrangedrelative to said chamber to be responsive to the pressure of fluidtherein and to disengage from said piston sealing means in response topressure in said chamber above a predetermined level.

16. The device of claim 15, further comprising stop means on saidhousing for contacting said piston sealing means to limit travel thereofin one longitudinal direction, said rod being forcibly movable fartherin said one direction than said piston sealing means to permitcorresponding forcible overtravel by said rod to said relative couplingposition while said piston sealing means are engaging said stop means,whereby upon forcing overtravel of the rod to said coupling position,said detent member frictionally engages said piston sealing means tosecure the latter to said rod.

g gi UNITED STATES PATENT OFFICE CERTIFICATE OF CO'RRECTIGN Patent No.3,775,027 Dated November 27, 1973 Inventor(s) ROGER L'. CRAFT I It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In Claim 6, line 1, the dependency reference numeral (1) should bedeleted, and thenumeral l substituted therefor.

Signed and sealed this 21st day ofMay 197g.

Si JAL) Attest EDWARD .FLETGIUER, JR. EFARSlLXLL DAii'i-L AttestingOfficer Cormissloner of Patents

1. A stepped displacement fluid device comprising: a housing providedwith a fluid pressure chamber and duct means for accommodating flow offluid to and from said chamber; piston means reciprocably mounted insaid chamber and having a working surface exposed to the pressure offluid in said chamber, said piston means being operable to reciprocatealone in said chamber when said chamber pressure is above apredetermined level for displacing fluid therefrom to said duct means inrelationship to the area of said exposed working surface; unitary pistonsealing means disposed about said piston means in sealing engagementwith the walls of said chamber; pressure actuatable coupling means forselectively engaging said piston sealing means to thereby effect saidcommon reciprocation, said piston sealing means being adapted to engagethe walls of said chamber to effect a static seal therebetween whilesimultaneously providing a dynamic seal between said piston and sealingmeans as said piston means reciprocates alone, and operable to engagesaid piston means to effect a static seal therebetween whilesimultaneously providing a dynamic seal between said sealing means andsaid chamber walls as said piston and piston sealing means reciprocatein comMon; and actuator means operatively connected to said piston meansfor reciprocating therewith during the reciprocation of said pistonmeans alone and during said common reciprocation of said piston andpiston sealing means.
 2. A stepped displacement fluid device comprising:a housing provided with a single fluid chamber and duct means foraccommodating flow of motive fluid to and from said chamber; pistonmeans reciprocably mounted in said chamber in sealing engagement withsaid housing for displacing fluid from said chamber in proportion to theworking area of said piston means exposed to pressure in said chamberand reciprocating therein, said piston means including first and secondmembers selectively reciprocal together in common movement to displacefluid in proportion to the sum of the working areas of said first andsecond members exposed to pressure in said chamber and operable to moverelatively to displace fluid in proportion to said exposed working areaof the first member; a single, pressure actuatable, selectivelyengageable coupling means on said piston means for operativelyinterconnecting said first and second members to effect said commonmovement thereof when said coupling means are engaged, said couplingmeans being the sole coupling force between said members and operable topermit said relative movement of the members when said coupling meansare disengaged; stop means on said housing for engaging said secondmember to limit movement thereof in one direction when engagedtherewith, said first member being positionable in a coupling positionrelative to said second member while the latter is in said stop-engagingposition to effect engagement of said coupling means and movablerelative to said second member away from said coupling position whilesaid second member is in said stop-engaging position to effectdisengagment of said coupling means; and actuator means operativelyconnected to said first member for moving therewith during both saidrelative movement and said common movement of said members.
 3. Thedevice of claim 2, wherein said second member is responsive to thepressure of fluid in said chamber to be biased thereby toward saidstop-engaging position, and wherein said coupling means comprise detentmeans associated with each of said first and second members operable tofrictionally intersecure said members for said common reciprocation uponpositioning said first member in said relative coupling position, saiddetent means being located at fixed positions on said first and secondmembers so as to disengage upon movement of said first member andassociated detenting lip away from said coupling position while saidpressure responsive second member and associated detenting lip remain insaid stop-engaging position.
 4. A stepped displacement fluid devicecomprising: a housing provided with a single fluid pressure chamber andduct means for accommodating flow of fluid to and from said chamber;first piston means reciprocably mounted in said chamber and having afirst working surface exposed to the pressure of fluid in said chamber,said first piston means being operable to reciprocate alone in saidchamber when said chamber pressure is above a predetermined level fordisplacing fluid therefrom to said duct means in relationship to thearea of said exposed first working surface; second pressure responsivepiston means reciprocably mounted in said chamber and having a secondworking surface exposed to said chamber pressure, said second pistonmeans being adapted to move in common reciprocation with said firstpiston means only when said chamber pressure is below said predeterminedlevel for displacing fluid to said duct means and performing work inrelationship to the sum of the areas of said first and second exposedsurfaces; a single, pressure actuatable coupling means carried by one ofsaid piston means for selectively interengaging said first and secondpistons when said chamber pressure is below said Predetermined level toeffect said common reciprocation, said coupling means being the solecoupling force between said pistons and acting in response to a rise insaid chamber pressure above said predetermined level to completelydisengage said first and second piston means; and actuator meansoperatively connected to said first piston means for reciprocatingtherewith during both said reciprocation of said first piston meansalone and during said common reciprocation of said first and secondpiston means.
 5. The device of claim 6, wherein said coupling meanscomprise a shoulder on said rod disposed to engage said sleeve to carrythe latter with said rod during one portion of travel of said rod toeffect said common reciprocation during said portion of rod travel, saidshoulder disengaging from said sleeve to allow said relativereciprocation of the rod during another portion of rod travel.
 6. Thedevice of claim 1, wherein said first piston means includes acylindrical rod having an inner end disposed in said chamber definingsaid first exposed working surface and an outer end extending outwardlyof said chamber defining said actuator means, and wherein said secondpiston means includes a cylindrical sleeve mounted to reciprocatelongitudinally in said chamber, said sleeve being positioned surroundingsaid rod and defining an annularly shaped second exposed workingsurface, said rod operable to reciprocate longitudinally relative tosaid sleeve during said reciprocation of said rod alone.
 7. The deviceof claim 6, wherein said second piston means further comprises a secondcylindrical sleeve defining an annularly shaped third working surfaceexposed to said chamber pressure, said second sleeve being operable inresponse to said chamber pressure to move in common reciprocation withsaid first sleeve when said chamber pressure is below a secondpredetermined pressure level lower than said first predeterminedpressure level to thereupon effect fluid displacement in directproportion to the sum of the areas of said first, second and thirdexposed surfaces.
 8. The device of claim 6, further comprising meanscarried by said first piston means for dynamically sealingly engagingsaid sleeve and rod to prevent fluid leakage therebetween during saidrelative reciprocation of the rod and operable to effect a static sealtherebetween during said common reciprocation of the rod and sleeve. 9.The device of claim 8 further comprising second sealing means carried bysaid second piston means for sealingly engaging the walls of saidchamber and said sleeve to prevent fluid leakage therebetween.
 10. Thedevice of claim 6, wherein said coupling means comprise noncompressibleand compressible detents located upon adjacent surfaces of said rod andsleeve and being interengageable to exert a predetermined friction forceinterconnecting said rod and sleeve to effect said common reciprocation,said detents being positioned relative to said chamber so that duringmovement of the rod, said chamber pressure effectively exerts a force onsaid detents of opposing said predetermined friction force and exceedingsame when chamber pressure is above said predetermined level tothereupon effect disengagement of said detents and permit said relativereciprocation of the rod.
 11. The device of claim 10 further comprisinga first stop member carried on said housing and located to engage saidsleeve to limit movement thereof in a first longitudinal direction, saidrod being movable in said first direction to a coupling positionrelative to said sleeve to effect interengagement of said detents whensaid sleeve is engaging said stop member.
 12. A stepped displacementfluid device comprising: a housing provided with a single fluid pressurechamber and duct means for accommodating flow of fluid to and from saidchamber; a cylindrical rod reciprocably mounted in said chamber and of adiameter to present a cylindrical space between the rod and the chamber,said rod being positiOned to travel longitudinally of said chamber toeffect displacement of fluid therefrom in proportion to said roddiameter; unitary piston sealing means of annular configuration disposedin said cylindrical space of the chamber for operatively engaging thewalls of said chamber and said rod to prevent fluid leakage from thechamber, said piston sealing means being selectively reciprocable incommon with said rod to effect fluid displacement from said chamber inproportion to said chamber diameter, said rod also being reciprocablerelative to said sealing means to effect said fluid displacementproportional to said rod diameter; and a single, pressure, actuatablecoupling means on said rod for selectively engaging said piston sealingmeans to thereby effect said common reciprocation thereof and said fluiddisplacement proportional to said chamber diameter upon rod travel, saidcoupling means being selectively disengageable from said piston sealingmeans to thereby effect said relative reciprocation thereof and saidfluid displacement proportional to said rod diameter upon rod travel,said piston sealing means being operable to statically and dynamicallyseal against said rod during said common and said relative reciprocationof said rod respectively.
 13. The device of claim 12, wherein saidpiston sealing means include a unitary fluid seal of annularconfiguration having an inner annular surface positioned in dynamic andstatic sealing engagement with said rod and an outer annular surfacepositioned in dynamic and static sealing engagement with said chamberwalls.
 14. The device of claim 12, wherein said coupling means comprisesa detent member carried on said rod operable to frictionally engage saidpiston sealing means to secure the latter to said rod upon positioningthe rod at a predetermined coupling position relative to said pistonsealing means.
 15. The device of claim 14, wherein said detent member isarranged relative to said chamber to be responsive to the pressure offluid therein and to disengage from said piston sealing means inresponse to pressure in said chamber above a predetermined level. 16.The device of claim 15, further comprising stop means on said housingfor contacting said piston sealing means to limit travel thereof in onelongitudinal direction, said rod being forcibly movable farther in saidone direction than said piston sealing means to permit correspondingforcible overtravel by said rod to said relative coupling position whilesaid piston sealing means are engaging said stop means, whereby uponforcing overtravel of the rod to said coupling position, said detentmember frictionally engages said piston sealing means to secure thelatter to said rod.